Measuring the Refresh Frequency of a Computer Monitor
Experiments from Team Labs

Experiment Profile

Summary

Students will gain a better understanding of frequency and the SI metric unit of Hertz by measuring, graphing, and calculating the refresh frequency of a computer monitor. Students will use the light probe sampling at 20,000 samples/sec to measure the light output of a computer monitor. They will measure the light output of the monitor over a very short period of time (100 milliseconds) to determine the rate at which the screen is turning on and off (Refresh Rate). From this data, they will examine the graph and calculate the refresh frequency using two different mathematical procedures.

Materials

Host Computer

ThinkStation SP16 Interface Kit

ThinkStation Interface

Power Supply

Communications Cable

Excelerator 2000 Software

Radiometric or Photometric Light Probe

Computer Monitor - CRT (This experiment will not work on an LCD or flat panel display such as a laptop)

Background

A computer monitor using a CRT (Cathode Ray Tube) has three electron guns. These guns fire electrons at red, green, or blue phosphor dots. When the electrons hit the phosphor dots, these dots emit light at specific wavelengths (red, green, blue). It is the various combinations of these three phosphor dots that produce all of the colors on the computer screen. The combination of one red, green, and blue phosphor dot creates a single pixel.

The image on the screen is continually updated, or "refreshed". The three electron guns sweep across the top of the screen in a single row and then move down to the next row and make another horizontal sweep. This continues down all of the horizontal rows on the computer screen. To give an example, a SVGA screen has 800 pixels in each horizontal row and 600 rows total. The electron guns start on the first row and sweep across all 800 pixels in that row. The guns then move to the next row and sweep across all 800 pixels in that row. This continues for all 600 rows before repeating. The rate at which the electron guns sweep across the screen is called the monitors horizontal refresh frequency. The rate at which the electron guns move down the screen is called the monitors vertical refresh frequency. In this experiment, students will be measuring the vertical refresh frequency of the computer monitor, which tends to be much slower than the horizontal refresh frequency.

Procedure

Collecting Data with the Light Probe

	1. Connect the ThinkStation Interface to your computer. Attach a Radiometric Light Probe to the interface.
Show me	2. Next, launch Excelerator 2000 and click on the Connect&GOTM icon. Excelerator will automatically identify the light probe and create a graph of irradiance vs. time. The software also sets a default sample rate and duration for the experiment that we will want to change.
	3. To change the sample rate and duration of the experiment, click on the Edit Clock icon located on the Excelerator toolbar.
Show me	Set the sample rate to 20,000 samples per second and the duration to 0.1 seconds.
	4. At this point, you are ready to record some data. Hold the light probe up against the computer screen.
	Click the green GO button on the left side of the Excelerator toolbar.
View screen	5. Excelerator will record the light emitted from the computer screen for exactly 0.1 seconds and will display a graph similar to the one at left. It may be necessary to click on Rescale in the Tools menu to see all of the data.

Analysis of the Data

	1. From the graph, it is possible to make a quick approximation of how fast the computer monitor is turning on and off. To do this, count the spikes on the graph and divide them by the total length of time. Remember, frequency is measured in Hertz (Hz), which corresponds to the number of cycles per second. In other words, how often is the computer monitor turning on and off in 1 second? In our graph above, we see 6 cycles in 0.1 seconds, which is equal to 60 cycles in 1 second or 60 Hz.
Show me	2. We can also calculate the refresh frequency more accurately by looking at the time between individual spikes as opposed to the number of spikes in a time interval. To do this, turn on your Select lines from the Analysis menu. Place one of the lines on a spike in the graph, and place the other line on the adjacent spike in the graph.
	3. Calculate the time interval between spikes. Using the data on the above graph as an example, 0.05849 sec - 0.04185 sec = 0.01664 sec. In other words, there is one cycle every 0.01664 seconds. This corresponds to a refresh frequency of 60.1 Hz.

Conclusions

To confirm the students understanding of frequency and how to calculate it from a graph, walk around the classroom and modify the refresh rates of their screens without them watching and then ask them to measure and calculate the new values using the light probe and Excelerator 2000.

Changing the Screen Refresh Frequency

Show me	1. The refresh frequency can be changed from the Display Properties dialog. Access this dialog by clicking the Start button on your Windows desktop, then Settings, Control Panel, Display, then the Settings tab. Change the refresh frequency and rerun the experiment.
	2. As another test of the students understanding of how to calculate frequency, provide them with the speed of their computers and ask them to calculate the time intervals between processor cycles. For example, a 500 Megahertz computer would have 2 nanoseconds between cycles. Solve for seconds:
	3. Since the students will have a difficult time relating to time measured in nanoseconds, have them calculate how far light will travel in that same amount of time. Since light travels at 3x108 m/s, in 2 nanoseconds light will travel 0.6 meters. In other words, light travels 0.6 meters during a single processor clock cycle in a 500 Megahertz computer. Students can compare this to the PC 1, which was manufactured by IBM in 1981. The PC 1 had a processor that ran at 4.77 MHz. How far would light travel during one processor clock cycle of the PC 1?

Extensions

Beyond measuring the refresh rate of their computer screen, students can use a similar technique with the light probe to measure the AC line frequency in their school. To do this, aim the light probe at a fluorescent light fixture in the ceiling and record data at 20,000 samples/sec for 0.1 seconds. Just like the computer monitor, which turns on and off, the fluorescent bulbs also flicker at a standard frequency. This flickering is caused by the alternating current, which runs through all of the wiring in their home or school. In North America, the AC line frequency is approximately 60Hz, whereas in Europe, the AC line frequency is 50 Hz. When students calculate the frequency from their graphs they will see a doubling of this frequency due to the fact that the lights get brighter twice per cycle.

About the author... Adam DiGiacomo is the Director of Curriculum at Team Labs. He is the author of several curriculum books, including the recently published "Probe-based Biology".

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